1. Field of the Invention
This invention relates to a frequency synthesiser for use, for example, in a communications receiver or transmitter such as a digital telephone.
2. Description of the Related Art
A GSM(Global System for Mobile Communication!) telephone, for example, may make use of a frequency synthesiser for switching between receive and transmit frequencies or for frequency hopping during either transmission or receiving. Such a frequency synthesiser will be required to hop over a 70 MHz bandwidth within one GSM time-slot (570 .mu.S). This is difficult to achieve as the loop bandwidth of the synthesiser needs to be kept to a minimum to maintain a low r.m.s. phase noise level.
Previous attempts have been made to speed up locking of the synthesiser loop by either temporarily widening the band width of the synthesiser loop or a voltage steering technique in which an external voltage is applied to the voltage input terminal of a VCO used in the synthesiser to drive the VCO frequency quickly to a value close to the required frequency. These prior proposals have, however, not been entirely successful, as they involve the introduction of additional active components (operational amplifier, voltage controlled amplifiers or others) inside the synthesiser loop.